[ORC] Extract and tidy up JITTargetMachineBuilder, add unit test.

(1) Adds comments for the API.

(2) Removes the setArch method: This is redundant: the setArchStr method on the
    triple should be used instead.

(3) Turns EmulatedTLS on by default. This matches EngineBuilder's behavior.

llvm-svn: 343423
diff --git a/llvm/unittests/ExecutionEngine/Orc/LazyCallThroughAndReexportsTest.cpp b/llvm/unittests/ExecutionEngine/Orc/LazyCallThroughAndReexportsTest.cpp
index 7caaa76..c3c5703 100644
--- a/llvm/unittests/ExecutionEngine/Orc/LazyCallThroughAndReexportsTest.cpp
+++ b/llvm/unittests/ExecutionEngine/Orc/LazyCallThroughAndReexportsTest.cpp
@@ -1,5 +1,5 @@
 #include "OrcTestCommon.h"
-#include "llvm/ExecutionEngine/Orc/ExecutionUtils.h"
+#include "llvm/ExecutionEngine/Orc/JITTargetMachineBuilder.h"
 #include "llvm/ExecutionEngine/Orc/LazyReexports.h"
 #include "gtest/gtest.h"